I appreciate the Hornet's suggestions, but I think Keijo's got it. I have to be really careful singing close up to my NTK through the Envoice, because of the sibilance, and nothing I've done in the mix can really remedy it satisfactorily.
If I had a channel strip with a de-esser (Presonus does one), I probably wouldn't have to worry about it, but I find repositioning the mic simpler and cheaper than buying the Presonus channel strip.